1. Field of the Invention
The present invention relates to polyurethane powder coating compositions that do not split off blocking agents and have a low stoving temperature, a process for their production, and their use for coating heat resistant substrates.
2. Description of the Prior Art
Under the pressure of ever more stringent environmental legislation, the development of powder coatings in addition to high-solids paints and aqueous coating systems has become increasingly important in recent years. Powder coatings do not release any harmful solvents during application, can be processed with a very high degree of material utilization, and are therefore considered to be particularly environmentally friendly and economic.
Qualitatively particularly high-grade, light-resistant and weather-resistant coatings can be produced with heat-hardenable powder coatings based on polyurethanes. The polyurethane (PUR) powder coatings currently established on the market are generally based on solid polyester polyols that are hardened with solid blocked aliphatic or cycloaliphatic polyisocyanates. These systems however have the disadvantage that the compounds used as blocking agents are split off during thermal crosslinking and largely escape. Accordingly, when they are processed special precautions have to be adopted, for technical reasons as well as for reasons of ecology and work safety, to purify the waste air and/or recover the blocking agent.
One possible way of avoiding the emission of blocking agents is to use the known PUR powder coating crosslinking agents containing uretdione groups (e.g. DE-A 2 312 391, DE-A 2 420 475, EP-A 0 045 994, EP-A 0 045 996, EP-A 0 045 998, EP-A 0 639 598 or EP-A 0 669 353). With these products the thermal reverse cleavage of uretdione groups into free isocyanate groups and their reaction with the hydroxy-functional binding agent is utilized as the crosslinking mechanism. However, in practice uretdione powder coating crosslinking agents have up to now been used only to a limited extent. The reason for this is the comparatively low reactivity of the internally blocked isocyanate groups, which requires stoving temperatures of at least 160° C.
Although it is known that the splitting-off of uretdione groups, in particular in the presence of reactants containing hydroxyl groups, occurs to a significant extent starting at about 100° C., the reaction in this temperature region still proceeds slowly, so that times of several hours, which are unrealistically long for a practical use, are necessary for the complete hardening of paint films. Although in DE-A 2 420 475, DE-A 2 502 934 or EP-A 0 639 598 temperatures starting at 110° C., and in DE-A 2 312 391 temperatures starting at 90° C., are mentioned as possible stoving conditions for uretdione group-containing powder coating systems, the specifically described examples of implementation show that sufficiently crosslinked coatings can also be obtained under reasonable stoving times of at most 30 minutes using the powder coatings described in these publications, but only starting at temperatures of 150° to 160° C. These publications do not give any information on how to prepare powder coatings that can in fact be completely hardened already at temperatures below 150° C. to 160° C. on a commercially feasible scale.
There has been no lack of attempts to accelerate the hardening of uretdione-crosslinking PUR powder coatings by the joint use of suitable catalysts. To this end various compounds have been proposed, for example, the organometallic catalysts known from polyurethane chemistry such as tin(II) acetate, tin(II) octoate, tin(II) ethylcaproate, tin(II) laurate, dibutyltin diacetate, dibutyltin dilaurate, dibutyltin maleate (e.g. EP-A 0 045 994, EP-A 0 045 998, EP-A 0 601 079, WO 91/07452 or DE-A 2 420 475), iron(III) chloride, zinc chloride, zinc 2-ethylcaproate and molybdenum glycolate; tertiary amines such as triethylamine, pyridine, methylpyridine, benzyldimethylamine, N,N-endoethylene-piperazine, N-methylpiperidine, pentamethyldiethylenetriamine, N,N-dimethylamino-cyclohexane and N,N′-dimethylpiperazine (e.g. EP-A 0 639 598); or N,N,N′-trisubstituted amidines, in particular bicyclic amidines such as 1,5-diazabicyclo[4.3.0]-non-5-ene (DBN) (e.g. EP-A 0 803 524).
Of these catalysts the aforementioned bicyclic amidines permit the lowest stoving temperatures. At the same time however they result in stoving yellowing (discolouration) that is unacceptably high for many areas of application. For this reason amidine-catalyzed uretdione systems have up to now not achieved a broad market penetration. In practice Lewis acids are generally used as catalysts, in particular organotin compounds of the aforementioned type. These compounds allow the formulation of uretdione powder coatings free of blocking agent that can fully react in a reliable and reproducible manner for example within 30 minutes at a temperature of 150° C. or, if shorter cycle times are desired, for example within 15 minutes at 180° C., to form coatings stable to yellowing and having good solvent resistance and elasticity.
Very special, complex powder coating formulations also permit a further reduction of the stoving temperature. According to the teachings of EP-B 1 137 689 Lewis acid catalysts, such as the previously mentioned tin or zinc compounds, are inhibited by acidic groups, such as carboxyl groups. Their full catalytic activity can therefore be manifested in a uretdione powder coating system only if the hydroxyl-functional binder that is used is free of carboxyl groups. For this reason a sufficient amount of an agent that is reactive to carboxyl groups, for example an epoxide, is added to the powder coatings described in this publication (which are based on conventional hydroxyl-functional binders, crosslinking agents containing uretdione groups, and special Lewis acid catalysts) in order to convert as completely as possible carboxyl groups that may still be present in the binder, and thereby remove them from the system. In this way the reactivity of the polyurethane powders can be increased to such an extent that the onset of hardening occurs starting at a temperature of about 120° C.
The increased reactivity of the acid-free catalyzed powder coatings is demonstrated in the examples of implementation of EP-B 1 137 689 exclusively by thin layer chromatography investigations. Results of paint-technology investigations are not disclosed. Our own experiments using real powder coating formulations confirm that, under the conditions mentioned in EP-B 1 137 689, i.e., using polyols having low acid numbers of at most 5 mg KOH/g and addition of a corresponding amount of an epoxide, crosslinked coatings can be obtained starting at a temperature of 120° C. However, these compositions exhibit a completely inadequate flow behavior, which is reflected in a strong surface structure and absence of gloss.
The possibility of increasing the reactivity of Lewis acid-catalyzed uretdione powder coatings by removing the inhibiting carboxyl group residue of the polyester resin by reaction with an epoxide, is also discussed in Metalloberfl{hacek over (a)}che, No. 55 (2001), 6, pp. 52-54. This publication also refers to the fact that although completely hardened paint films can be obtained by using commercial powder coating resins and curing agents at a temperature of 130° C. and a stoving time of 30 minutes, they exhibit poor flow behavior. The unsatisfactory flow is in this case attributed to the use of commercial raw materials, which have been specially developed for stoving temperatures above 160° C. and are therefore not ideally suitable for low temperature applications.
An object of the present invention is to provide new PUR powder coatings that do not undergo splitting-off of blocking agents and are based on readily available, conventional binder components, that harden at just as low stoving temperatures and correspondingly short stoving times as the systems of EP-B 1 137 689 and therefore provide fully crosslinked paint films, but which despite the high reactivity exhibit very good surface properties, in particular an excellent flow behavior.
This object was achieved by the zinc-catalyzed uretdione powder coatings based on binders with a defined minimum content of carboxyl groups.
The present invention is based on the surprising observation that by using PUR powder coatings that do not undergo splitting-off of blocking agents and are based on uretdione powder coating crosslinking agents, conventional hydroxyl-functional binders, zinc catalysts and an at least equimolar amount, based on the carboxyl groups present in the system, of a compound reactive to carboxyl groups, elastic and solvent-resistant coatings having outstanding flow behavior can be obtained at temperatures starting at 110° C. if powder coating polyols with acid numbers of at least 6 mg KOH/g, i.e., with comparatively high residual contents of carboxyl groups, are used as binder; whereas, the use of binders free from or with low carboxyl group contents leads under otherwise identical conditions to the considerable flow disorders described above.
Although the presence of carboxyl groups reduces the catalytic activity of Lewis acids on uretdione crosslinking and the carboxyl group content thus has a significant influence on the reactivity and consequently the crosslinking temperature, the powder coating binders described in EP-B 1 137 689 are defined exclusively by their content of OH groups. There are no details of permitted maximum or indeed necessary minimum contents of carboxyl groups in the reaction system. The examples of implementation describe as binder component simply a polyester polyol with an acid number≦5 mg KOH/g as well as a practically acid-free polycaprolactone, and thus suggest the use of binders with as low an acid number as possible.